There is much more to acceleration than just the engine. There is weight and gearing... both are hugely important.

Power and weight, specifically their ratio is the single most important factor is a vehicles straight line performance. I used to think gearing mattered more than it does. Gearing certainly must be matched the the cars torque and redline, but once that is done the vehicles performance is not too dependent on individual ratios and spacing, etc.

The M3 appears to be a bit of an overachiever given its power and weight. I think part of that comes from its chassis, suspension and tires. All critical in terms of hook up and getting more acceleration and less wheel spin, wheel hop, etc. The car also appears to have a very efficient drive train (low parasitic losses). However, putting to bed the issue of a small underrating vs. a highly efficient drive train is extremely challenging and that absolutely won't be solved with any normal dyno testing.